Normal Weight Obesity – a myth or a reality?

Introduction

There is a widely prevalent myth amongst the populace that normal body weight always equals healthy weight and they do not need to worry about practising healthy lifestyle behaviours. However, nothing could be further from the truth. Normal weight obesity is well recognised entity. In my post “What is obesity – is it merely about BMI?” What is obesity? obesity had been defined as “excessive body fat accumulation (not weighing too much), which is associated with clear risks to health.” Surprisingly, even though obesity has been defined as the presence of ‘excess’ fat, there is no consensus on how to define obesity based on body fat content or body fat percentage.

WHO has established Body Mass Index (BMI) as the parameter for identifying overweight and obesity. Because of its simplicity, it has become a popular tool for assessing the prevalence of obesity and overweight at the population level. However, BMI has some serious limitations which will be discussed in detail in a subsequent post. The main limitation of BMI is that it cannot differentiate body fat from lean (fat-free) mass and central (intra-abdominal fat or visceral fat; colloquially known as belly fat) from peripheral fat. As a result, current BMI criteria miss more than half of the individuals with increased body fat percentage, who would otherwise be categorised as ‘obese’ using the WHO criteria of excess body fat, especially in older adults.

In my post referred to above, I had described that it is the visceral fat (intra-abdominal fat or belly fat) which determines the risk of various obesity-related diseases conditions. Therefore, in this post, I will also be addressing a very common concern of the people – how to reduce belly fat? or how to lose belly fat?

Classification of obesity based on body fat percentage

As has been highlighted above, there is no universally accepted definition of obesity based on total body fat mass calculation or body fat percentage. The task has been complicated by the distinct tendency for percentage body fat to steadily increase with advancing age. In addition, the total body fat and its distribution in the body is influenced by sex and ethnicity. The mechanisms that lead to increased body fat with age are not clearly understood. In general, body fat for young adult men averages between 12 and 15 per cent; the average value for women falls between 25 and 28 per cent. Based on these averages, different investigators have proposed different cut off values for per cent of body fat to define excess fatness. The different cut off points for body fat vary between 20 and 25 per cent for men and 30 and 35 per cent for women. The textbook of ‘Exercise Physiology: Nutrition, Energy, and Human Performance’ proposes the criterion of above 20 per cent for men and above 30 per cent women, to determine obesity. American Society of Endocrinologists has defined obesity by body fat per cent as greater than 25 per cent in men and greater than 35 per cent in women.

What is Normal Weight Obesity?

The definition of obesity has become a dynamic concept and has been evolving over time as our understanding of adipose tissue (body fat) continues to evolve. In my post “Is obesity a disease or a risk factor for other conditions?” Is obesity a disease? I had highlighted that fat cells are a veritable endocrine factory and secrete a variety of metabolic, hormonal, and inflammatory products. These substances along with ectopic fat (storage of fat in tissues other than adipose tissue that normally contain only small amounts of fat, such as the liver, skeletal muscle, heart, and pancreas) give rise to a number of disease conditions.

In view of the recognition of excessive body fat as a major risk factor for the development of various lifestyle diseases (chronic or noncommunicable diseases), some researchers suggested the need for measurement of body fat content and information on body fat distribution for diagnosis of obesity at the individual level. In the year 2006, De Lorenzo and colleagues coined the term ‘Normal Weight Obesity’ (NWO) to describe those individuals who have weight within normal limits according to the BMI but have a high body fat percentage.

Does Normal Weight Obesity predispose to increased health risks?

In my post “What is obesity – is it merely about BMI?’, referred to above, I had described a subset of individuals within individuals classified as obese, who despite having an excess of body fat have a ‘normal metabolic profile’. This subset of individuals has been labelled as ‘metabolically healthy obese’ (MHO), in contrast to obese individuals who present with deranged metabolic parameters and have been labelled as ‘metabolically unhealthy obese’ (MUO). Similarly, not everybody with normal weight obesity presents with metabolic abnormalities or disease conditions associated with excess body fat. Within normal weight obesity, there is a subset of people who show a high degree of abnormalities in the regulation of metabolic processes. Ruderman and colleagues coined the term ‘metabolically obese normal weight’ (MONW) to describe this subset of individuals.

The MONW subjects have an appropriate weight according to BMI but exhibit greatly increased risk for metabolic syndrome. These individuals usually have excessive visceral fat (intra-abdominal or belly fat), hyperinsulinemia (hyper ~ over or excess; insulin ~ a hormone which regulates blood sugar. A condition in which there are excess levels of insulin circulating in the blood relative to the level of glucose. It increases the risk for higher triglyceride levels, high uric acid, atherosclerosis [narrowing of the arteries caused by a buildup of plaque], weight gain, hypertension, and type 2 diabetes), insulin resistance (reduced sensitivity of the body to the hormone insulin, resulting in high blood sugar levels), dyslipidemia (abnormally elevated blood cholesterol), high blood pressure, and an increased risk for type 2 diabetes, cardiovascular events (heart attack, stroke etc), and cardiovascular death.

Metabolic syndrome is a cluster of metabolic disorders, which increases the chances for future cardiovascular diseases more than any one factor presenting alone. Metabolic syndrome occurs when a person has at least three of the five following medical conditions: abdominal obesity (visceral or belly fat), high blood pressure, high blood sugar, high serum triglycerides and low high-density lipoprotein [HDL] (good cholesterol) levels.

In normal weight obesity, these metabolic disorders are present at a lesser magnitude, which usually remain undiagnosed. Moreover, abdominal obesity (visceral fat; belly fat) may not be always present. As a result, in contrast to MONW subjects, individuals with normal weight obesity are often unaware of the risk to which they are exposed. Over time, however, individuals with normal weight obesity might eventually present as being metabolically obese, while continuing to remain normal weight. Thus, rather than being two separate entities, they can be considered as two ends of a continuum.

Factors responsible for metabolic dysregulation in MONW subjects.

Excessive body fat is now a well established major risk factor for the development of various lifestyle diseases (chronic or noncommunicable diseases) as it results in impaired regulation of metabolic processes. It is becoming increasingly evident that all fat is not bad. There is convincing evidence to prove that visceral fat (intra-abdominal or belly fat) is a risk factor for the development of various lifestyle diseases, regardless of the total body fat content. Even in individuals with normal BMI, increasing abdominal obesity (increased visceral or belly fat) is linked to higher mortality in adults with or without existing coronary heart disease (CHD; a disease condition resulting from narrowing of blood vessels [coronary arteries] supplying blood to the heart; it is the usual underlying cause of a heart attack).

Some studies have reported that among patients with existing coronary artery disease (damage or disease in the heart’s major blood vessels) ‘normal weight with abdominal obesity’ (increased visceral or belly fat) is associated with the highest risk of mortality compared with subjects with ‘normal BMI and no abdominal obesity’ and with ‘obese patients by BMI regardless of their abdominal obesity’ status.

Prevalence of Normal Weight Obesity

Prevalence data for normal weight obesity differs between various research studies because of the ethnic differences in total body fat and body fat distribution and the lack of universally accepted definition of normal weight obesity. According to an article “The Obese Without Cardiometabolic Risk Factor Clustering and the Normal Weight With Cardiometabolic Risk Factor Clustering: Prevalence and Correlates of 2 Phenotypes Among the US Population (NHANES 1999-2004)” published in the Journal of the American Medical Association in 2008, 23.5 per cent of normal weight US adults ages 20 years or older were metabolically abnormal. And such individuals account for an even a bigger percentage of the population in Asian countries. In my post “What is obesity – is it merely about BMI?”, referred to above, I had described a characteristic obesity phenotype (an individual’s observable characteristics) – Asian Indian Phenotype or Paradox, which is characterised by abdominal obesity (increased visceral or belly fat) but relatively lower BMI. According to Faidon Magkos, PhD, a self-described metabolist, who holds a joint appointment at the Singapore Institute for Clinical Sciences and the Yong Loo Lin School, in India 40 per cent of people considered to be normal weight by virtue of their body mass index (BMI) have metabolic dysfunction, such as high blood glucose and high blood triglycerides.

Management of Normal Weight Obesity

In view of the major role played by visceral fat (intra-abdominal or belly fat) in the causation of various obesity-related diseases and disorders, more people seem to be concerned about how to reduce belly fat? or how to lose belly fat? so as to ameliorate obesity-related disorders.

Spot reduction

Spot reduction refers to the fallacy that fat can be targeted for reduction from a specific area of the body through the exercise of specific muscles in the desired area. This notion arises from the belief that an increase in the metabolic activity of the muscle will result in greater utilisation of the fat overlying the muscle or in close proximity to the active muscle, for energy supply. One of the most common myths related to spot reduction is that performing a large number of crunches and sit-ups can reduce excessive abdominal and hip fat.

Though the promise of spot reduction with physical activity seems attractive, from both aesthetic and health risk standpoint, unfortunately, the scientific consensus is that spot reduction is a myth. The negative energy balance created through regular physical activity contributes to reducing total body fat. It does not cause preferential reduction of fat pads directly over the active muscles. This is because physical activity stimulates the utilisation of fatty acids via hormones and enzymes that act on fat depots throughout the body.

In view of the deleterious role of visceral fat (intra-abdominal or belly fat) in the causation of a variety of health problems, and the belief that its reduction may ameliorate obesity-related disorders, how to reduce belly fat or how to lose belly fat has been a key area for research. Investigators have studied diet and/or physical activity, weight-loss promoting drugs and bariatric surgery to look for a method which can preferentially target visceral fat (intra-abdominal or belly fat). A review article “Subcutaneous fat loss is greater than visceral fat loss with diet and exercise, weight-loss promoting drugs, and bariatric surgery: a critical review and meta-analysis” was published in the International Journal of Obesity, in Mar 2017. The article concluded – “the main finding of this review is that there is no evidence of a weight loss intervention (lifestyle modification, weight-loss promoting drugs, bariatric surgery) that preferentially targets visceral adipose tissue (VAT). Therefore, there are no shortcuts to losing VAT with the available strategies.”

How to reduce belly fat?

In my post “What is obesity – is it merely about BMI?” I had discussed various factors, including genetic susceptibility, age, gender, and ethnicity, which predispose to preferential accumulation of visceral fat (intra-abdominal or belly fat). Though you cannot change many of these risk factors, there are several ways you can follow to minimise the accumulation of visceral fat (intra-abdominal or belly fat). Also, the good news is that because visceral fat (intra-abdominal or belly fat) is metabolically more active, it responds more efficiently to diet and exercise than fat on hips and thighs. The review article published in the International Journal of Obesity referred to above, states – “Considering all studies together, per cent change of VAT is greater than per cent change of subcutaneous adipose tissue (SAT) and fat loss is proportional to BMI loss.”

Some approaches that may help in the management of NWO and keep visceral fat (intra-abdominal or belly fat) at bay are as under:

Weight loss

In case of overweight and obese individuals, a straightforward advice involves lifestyle modifications to lose weight. But what about patients with metabolic dysfunction whose BMI falls within normal range? Could they also benefit from losing weight? In obese people, benefits of weight loss as small as 5 per cent of their baseline weight are well documented. However, data on the benefits of weight loss in MONW subjects are limited due to various reasons. The main reason is that it is difficult to recruit people for such studies as people who are of normal weight don’t think that they need to lose weight. At the beginning of my post, I had highlighted a widely held myth ‘normal weight always equals healthy weight’.

Magkos and colleagues carried out a study on a small number of MONW subjects. The study found that small amount of weight loss, approximately 7 pounds on average, had multiple beneficial effects, not only on total body fat but also on visceral fat (intra-abdominal or belly fat) and liver ectopic fat; it also improved various metabolic parameters like plasma insulin, triglycerides and total cholesterol. However, even though diet-induced weight loss was effective in treating metabolic dysfunction in MONW individuals, “even small amounts of weight loss may not be a feasible or optimal recommendation, especially in older MONW subjects.

Exercise

Exercise, whether you are metabolically healthy or unhealthy, will improve your metabolic health. In addition, it is evident that exercise in sufficient amounts can lead to a substantial decrease in body weight, total body fat and visceral fat (intra-abdominal or belly fat). Additionally, there is evidence to support a dose-response relationship between exercise amount and these. i.e. more exercise leads to additional benefits. However, exercise has not been shown to cause a ‘preferential’ reduction of visceral fat (intra-abdominal or belly fat). To derive optimum benefits, exercise program must encompass all components of physical fitness, including strength training and should follow principles of exercise prescription. After all ‘exercise is medicine’. I will cover components of physical fitness and principles of exercise prescription in my subsequent post.

Healthy eating

Choose a balanced diet that helps you achieve and maintain a healthy weight. Healthy eating entails not just focussing on the quantity but also on the quality of calories consumed. Healthy eating plays an important role in reducing the risk of lifestyle diseases and death in MONW individuals.

Do not smoke

In my post “What is obesity – is it merely about BMI” I had pointed out that smoking is responsible for the ‘preferential’ accumulation of visceral fat (intra-abdominal or belly fat). The more you smoke, the more likely you are to store fat in your abdomen.

Conclusion

Normal weight doesn’t always equal healthy weight. As our understanding of adipose tissue has evolved, newer phenotypes of obesity – Normal Weight Obesity and MONW have been identified. Using the term ‘normal weight obesity’ is really a way of being more precise about the changing conceptualisation of obesity, because the real definition of obesity is ‘excess body fat’. However, because of the limited obesity classification, which is solely based on BMI, individuals with normal weight obesity are seldom identified through routine healthcare. Given that excessive body fat is associated with a higher risk of cardiovascular diseases ( disease conditions related to the heart and blood vessels such as heart attack, stroke, heart failure etc) and other associated health conditions, accurate and timely diagnosis of normal weight obesity is paramount. Also, in the light of a lack of consensus, there is an urgent need to establish appropriate diagnostic criteria for normal weight obesity.

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